1. Field of the Invention
The present invention relates to a steering rack shaft in which strain is reduced by partial thermal refining, and which facilitates the machining of rack teeth and the like, improves the manufacturing efficiency, lowers the manufacturing cost, and has enhanced rigidity, toughness, and strength, and to a method of manufacturing the same.
2. Description of the Related Art
A generally known steering rack shaft is thermally refined after a workpiece prior to the formation of rack teeth is quenched, the rack teeth are formed after the thermal refining, and the portion where the rack teeth have been formed is requenched, thereby imparting wear resistance characteristics to the steering rack shaft. The conventional thermal refining is considered to be performed over the entire body of the workpiece, i.e., over the region of its entire length.
Thermal refining referred to herein is an operation by which a workpiece, after being quenched, is tempered to a relatively high temperature (approx. 400.degree. C. or more) so as to be converted to a troostite structure or a sorbite structure. The quenching treatment in this thermal refining is aimed at hardening or strengthening the workpiece up to its central portion uniformly, and after quenching, high-temperature tempering to an appropriate temperature of approximately 400.degree. C. or more is usually conducted.
However, in a case where whole thermal refining is performed as in the case of a conventional steering rack shaft, there has been a drawback in that the strain (deflection) of the shaft due to that thermal refining becomes large in the form of a bow, as shown by the dotted line representing the conventional case in the graph of comparison of strain lines of heat treatment in FIG. 23. For this reason, much time is required for the strain-relieving operation, so that there has been a drawback in that the manufacturing efficiency declines, thereby resulting in higher manufacturing cost. Furthermore, even if the strain caused by the whole thermal refining is corrected, there has been a drawback in that a large strain occurs each time heat treatment is subsequently provided (for a rack teeth portion, a shaft portion, and the like) due to the residual stress in the workpiece (the steel product of the steering rack shaft).
In addition, it has been necessary to set the hardness in thermal refining to a relatively low level so as to improve the machining efficiency and the life of the machine tool by ameliorating the machinability of the rack teeth at the rack teeth-forming portion and the like. For this reason, it has been difficult to improve the strength and toughness of the steering rack shaft and to enhance the rigidity thereof. That is, if the hardness of the inner portion is reduced to ameliorate the machinability as shown in FIG. 24 (the relationship between the hardness of the inner portion of the shaft and bending load), the resistance against a bending load becomes weak. On the other hand, if the resistance against the bending load is strengthened, and the harder the workpiece (steering rack shaft) is thereby made, the more the following disadvantages appear such as the acceleration of damage to the tool, the deterioration of machining accuracy, increased cutting resistance, increased cutting temperature, and the promotion of progress of wear.
Furthermore, the whole thermal refining in a furnace is a method which employs a facility in which an in-furnace temperature is set to a high level, and heat treatment is provided with respect to a multiplicity of workpieces in one lot. This method is quite different from a material handling process in which workpieces can be processed or handled one at a time as in other machining or high-frequency induction hardening. Hence, the preparation and machining of the workpieces in this method need to be effected at a number of places and require special management of the workpieces therefor. For this reason, in thermal refining treatment, much labor is required including the operation of placing a multiplicity of workpieces within the furnace. As a result, there have been drawbacks in that the number of manufacturing processes increases, that the material handling becomes complicated, and that it is difficult to adopt an integrated manufacturing line, leading to a decline in the manufacturing efficiency and an increase in the manufacturing cost. In addition, if the whole thermal refining of a multiplicity of workpieces is used, it is impossible to circumvent the possibility of occurrence of a multiplicity of defects in quality at one time.